1. Field of the Invention
This invention relates to the field of IMPATT diodes, and particularly to Read-type double drift IMPATT diodes.
2. Description of the Prior Art
IMPATT diodes have found extensive use in solid-state microwave components due to their excellent power-generating capabilities at C-band and higher frequencies. The performance of IMPATT power sources has advanced rapidly as continuing technology improvements have resulted in diodes with higher conversion efficiencies and output power. Technological problems favored the use of Si in the early stages of IMPATT development. However, promise of higher efficiency prompted intensive efforts to develop GaAs IMPATT technology. These efforts have culminated with the development of Read-type device structures with high-low and low-high-low doping profiles. Such devices have advanced the state-of-the-art significantly by exhibiting conversion efficiencies greater than 30% at X band frequencies.
The advantage of Read-type devices can be understood in terms of the basic operating principles of IMPATT diodes. A thin avalanche region is needed to inject an inductively delayed charge pulse into a drift region. In a double drift structure, separate drift regions are provided for holes and electrons. The further transport delay in the drift regions results in a negative resistance. Ideally, the carriers should drift at scattering-limited velocities in the drift regions, which requires a certain minimum field to be maintained during the RF cycle. This minimum drift field is typically one to two orders of magnitude lower than the breakdown field. Thus, the avalanche voltage will contribute significantly to the operating voltage (typically 30-50% ) in spite of the avalanche region being thin.
The large avalanche voltage is a major factor in compromising the efficiency of IMPATT diodes because the efficiency of a diode is limited to first order by its voltage as follows: ##EQU1## where: V.sub.D =drift voltage, and
V.sub.A =avalanche voltage.
Prior art double drift IMPATT diodes are created by reverse biasing a p-n junction that is formed in a single semiconductor such as Si or GaAs. The efficiency obtained by such homojunction diodes can be increased utilizing various doping profiles to optimize the structure. However, the device performance is ultimately limited by the inherent physical properties of the material, particularly the band gap energy and the carrier mobilities. The avalanche voltage is directly proportional to the bandgap energy while the series resistance of the diode is inversely proportional to the carrier mobility in the drift region. The prior art use of a single material to construct double drift IMPATT diodes greatly limits design flexibility and the capability of the diode. For example, Ge has an energy gap of 0.72 eV which is low compared to the energy gap of 1.40 eV for GaAs. Hence, a Ge diode requires a lower avalanche voltage than a GaAs diode. However, Ge has an electron mobility of only 3900 cm.sup.2 /V sec compared to GaAs electron mobility of 8500 cm.sup.2 /V sec. Consequently, Ge homojunction IMPATT diodes suffer from their relatively low electron drift mobility as compared to GaAs.